What is the recommended way to copy an entire S3 bucket in Step Functions, including large files, so that size limits and timeouts won't cause problems?

S3 buckets can have a virtually unlimited size. This means the copy operation could take some time. To perform an S3 bucket to bucket copy asynchronously, I would look at using a Lambda function to call an S3 batch operation in your StepFunction.

S3 Batch Operations are an Amazon S3 data management feature that lets you manage billions of objects at scale with just a single API request. More on S3 Batch Operations here . . .

https://aws.amazon.com/s3/features/batch-operations/